Gemini Flash: one malformed MCP array enum breaks all prompts with HTTP 400

Type: Bug

Copilot CLI Version: 1.0.84-5
OS Version: Windows 11 x64
Model: gemini-3.7-flash

Summary

Having a single MCP tool whose schema contains an integer enum directly on an array property causes all requests to Gemini Flash to fail with an opaque HTTP 400 Bad Request error.

The error does not identify which MCP server, tool, or schema property caused the failure.

Steps to Reproduce

1. Save schema-repro.mjs, gemini-bad.json, and gemini-fixed.json from the Reproducer files section below in the same directory. Node.js must be available.
2. From that directory, run these native Copilot commands:

copilot --model gemini-3.7-flash --no-custom-instructions --additional-mcp-config "@gemini-bad.json" --available-tools schema_repro-probe_000 --allow-all-tools -p "Reply with exactly OK. Do not call tools."
copilot --model gemini-3.7-flash --no-custom-instructions --additional-mcp-config "@gemini-fixed.json" --available-tools schema_repro-probe_000 --allow-all-tools -p "Reply with exactly OK. Do not call tools."

Both commands advertise exactly one inert tool. There is no SDK dependency or shell helper.

The bad fixture defines:

{
  "type": "object",
  "properties": {
    "levels": {
      "type": ["array", "null"],
      "items": { "type": "integer" },
      "enum": [0, 1, 2, 3, 4, 25]
    }
  }
}

Actual Behaviour

The request immediately fails with:

Execution failed: 400 Bad Request (Request ID: ...)

Moving enum under items:

{
  "type": ["array", "null"],
  "items": {
    "type": "integer",
    "enum": [0, 1, 2, 3, 4, 25]
  }
}

completely resolves the issue and the request succeeds.

Native CLI results:

Expected Behaviour

Copilot CLI should validate tool schemas client-side before sending them to the provider API, and report actionable errors identifying:

• Server name
• Tool name
• Schema property path

For this fixture: schema_repro, probe_000, and /properties/levels/enum.

Failing that, the error message should include the provider's validation details instead of a generic 400 error.

Notes

• Other schema constructs like plain integer enums ({"type": "integer", "enum": [0, 1]}) and nullable objects work fine.
• Related to #4623, but specifically caused by array-level enums rather than nullable object unions.

Impact

A single malformed tool schema from any active MCP server silently breaks all model calls to Gemini without any indication of which server or tool is at fault.

import { createInterface } from 'node:readline';

const mode = process.argv[2];
const count = Number(process.argv[3] ?? 1);
const allowedModes = ['array-enum-bad', 'array-enum-fixed', 'count'];

if (!allowedModes.includes(mode) ||
    !Number.isInteger(count) ||
    count < 1 ||
    count > 400) {
  throw new Error('Usage: node schema-repro.mjs array-enum-bad|array-enum-fixed|count [1..400]');
}

const levels = { type: ['array', 'null'], items: { type: 'integer' } };
if (mode === 'array-enum-bad') {
  levels.enum = [0, 1, 2, 3, 4, 25];
} else {
  levels.items.enum = [0, 1, 2, 3, 4, 25];
}

const tools = Array.from({ length: mode === 'count' ? count : 1 }, (_, index) => ({
  name: `probe_${String(index).padStart(3, '0')}`,
  description: 'Schema diagnostic only; performs no operation.',
  inputSchema: {
    type: 'object',
    properties: mode === 'count' ? { value: { type: 'string' } } : { levels },
  },
}));

for await (const line of createInterface({ input: process.stdin, crlfDelay: Infinity })) {
  if (!line.trim()) {
    continue;
  }
  const request = JSON.parse(line);
  if (request.id === undefined) {
    continue;
  }

  let result;
  if (request.method === 'initialize') {
    result = {
      protocolVersion: request.params.protocolVersion,
      capabilities: { tools: {} },
      serverInfo: { name: 'schema-repro', version: '1.0.0' },
    };
  } else if (request.method === 'tools/list') {
    result = { tools };
  } else if (request.method === 'tools/call') {
    result = {
      isError: true,
      content: [{ type: 'text', text: 'This diagnostic never executes an operation.' }],
    };
  } else if (request.method === 'ping') {
    result = {};
  } else {
    process.stdout.write(`${JSON.stringify({
      jsonrpc: '2.0',
      id: request.id,
      error: { code: -32601, message: 'Method not found' },
    })}\n`);
    continue;
  }

  process.stdout.write(`${JSON.stringify({ jsonrpc: '2.0', id: request.id, result })}\n`);
}

{
  "mcpServers": {
    "schema_repro": {
      "type": "stdio",
      "command": "node",
      "args": ["schema-repro.mjs", "array-enum-bad"],
      "tools": ["*"]
    }
  }
}

{
  "mcpServers": {
    "schema_repro": {
      "type": "stdio",
      "command": "node",
      "args": ["schema-repro.mjs", "array-enum-fixed"],
      "tools": ["*"]
    }
  }
}

Start with schema-repro.mjs, gemini-bad.json, and gemini-fixed.json, then run the two native Copilot commands in the reproduction steps to confirm the failing and passing cases. Trace where MCP tool schemas are handled before the Gemini request is sent. Done means malformed array-level enums produce an actionable error naming schema_repro, probe_000, and /properties/levels/enum, or expose the provider's validation details instead of a generic 400.